In modern flexographic liquid application systems, a transfer roll such as an anilox roll is wetted by a liquid, for example ink or an adhesive, and is rotated to apply the liquid to the surface of an adjacent rotating plate roll. The plate roll has a a rubber surface which is shaped or engraved to pick up liquid from the transfer roll and to deposit the liquid in a desired pattern on the surface of an adjacent moving web of material.
In a gravure system, the transfer roll is an engraved printing cylinder which transfers liquid in a desired pattern directly onto the surface of a web of material. In such known systems, the transfer roll picks up the wetting liquid from a trough or "fountain" and rotates to meter a desired amount of liquid onto the adjacent plate roll or web of material.
Rapidly rotating transfer rolls in known apparatus typically fling excess liquid radially and axially of the transfer roll during the liquid application process. It is known to utilize fluid deflectors or "slingers" at the ends of the transfer roll to direct at least a portion of the liquid flung from the roll back into the underlying trough or fountain. A substantial amount of liquid is lost in this process and liquid is also easily splashed or sprayed over the driving mechanism of the transfer roll or even over the web. Also, the liquid in the trough may thicken as a result of evaporation or contamination.
Moreover, as the rotational speed of the transfer roll increases to, for example, 800 feet per minute, the roll may be unevenly wetted. In operation, this results in a "starvation" phenomenon wherein ink or other liquid is unevenly transferred to the web and printing or coating is therefore incomplete.
In the U.S. Pat. No. 2,931,297 to E. A. Coudriet, entitled Ink Seal Means For Rotary Intaglio Printing Mechanism, a printing apparatus is disclosed which allows relatively high speed wetting of a web, for example at about 1,000 feet per minute. This relatively high speed operation is attained by utilizing a doctor blade assembly in association with a reservoir chamber through which wetting liquid is pumped. The reservoir chamber is sealed at its ends to block the axial slinging of liquid and to thus allow relatively even wetting of the transfer roll at relatively high rotational speeds. This system has the added advantage of maintaining a relatively constant flow of liquid within the reservoir, thus avoiding problems associated in the prior art with contamination of wetting liquid and drying and thickening of stagnant liquid in a fountain.
An end sealed reservoir can also provide a relatively even wetting of a transfer roll, even if the viscosity of the wetting liquid changes. In prior systems changes in viscosity of the wetting liquid would cause corresponding undesirable changes in the uniformity of wetting of the transfer roll.
The reservoir end seals of the Coudriet patent are formed by relatively resilient sealing elements which are pressed against the rotating surface of a transfer roll to seal the reservoir against the roll. In operation, these resilient seals rub against the moving surface of the roll and must therefore be changed frequently in order to maintain a fluid seal. Replacing worn end seals requires a substantial amount of time and therefore reduces the speed advantage obtained by providing an end sealed reservoir. Also, the end sealed apparatus must operate with a special transfer roll which has relatively smooth sealing areas at its ends. The end sealed reservoir structure therefore provides some advantages over prior structures but also has the disadvantage of requiring a special transfer roll and relatively frequent changes of its sealing components.
It has been suggested that an advantageous liquid transfer operation may be achieved in an apparatus utilizing a single doctor blade by resting opposite ends of the doctor blade upon stationary rings that are mounted in spaced relation to the ends of a rotating transfer roll. In operation, liquid accumulated between the ends of the roll and the adjacent rings is retained by capillary attraction and is therefore not flung from the roll. This apparatus has the disadvantage of utilizing a trough or fountain, with the associated problems of liquid contamination and stagnant liquid in the fountain. Moreover, the apparatus has the above-described fluid starvation problems and is therefore not suitable for relatively high speed operation.
Accordingly, it is an object of the invention to provide a high speed liquid transfer apparatus which employs a reverse doctor blade assembly with a liquid reservoir having stationary end seals which seal the reservoir in fluid communication with the rotating surface of a transfer roll.
A further object of the invention is to provide such stationary end seals with resilient sealing elements which sealingly engage seal support elements mounted in stationary sliding relation to the ends of the transfer roll.
In liquid transfer systems which employ a sealed reservoir to apply liquid to a rotating transfer roll it is important to maintain a constant, relatively even level of wetting liquid within the reservoir and a constant flow of liquid through the reservoir. A constant flow is required to avoid settling of constituents of the liquid, particularly when water base inks are used. A constant supply of fresh liquid at a modest flow rate is also useful in eliminating or reducing foaming of the liquid within the reservoir and in minimizing the volume of liquid required to supply the reservoir.
Accordingly, it is a further object of the invention to provide a liquid transfer apparatus with a doctor blade assembly which includes a closed reservoir with a central supply inlet and vertically adjustable end outlets for removing liquid from the reservoir to maintain a constant level of liquid within the reservoir and a constant flow of liquid through the reservoir.
It is another object of the invention to provide an apparatus with a reduced volume of liquid flowing through the reservoir in order to minimize the total volume of liquid required for circulation and to thereby increase the efficiency and reduce the cost of operation of the apparatus.
It is important in manufacturing to increase efficiency by reducing the amount of time required to replace worn doctor blades.
Accordingly, it is another object of the invention to provide a doctor blade assembly which may be easily and quickly operated to change doctor blades.
The doctor blade assembly of the invention provides a relatively even wetting for webs travelling at 2000 or more feet per minute. It has been found that such high speed operation can cause an undesirable turbulent cascading of liquid within the reservoir when the doctor blade shears excess liquid from its transfer roll. The turbulent cascading of liquid within the reservoir causes an uneven wetting of the rapidly moving transfer roll and thus reduces the reliability of the system.
Accordingly, it is an object of the invention to provide an end sealed reservoir of a reverse doctor blade assembly with a shape which reduces and controls turbulent flow of wetting liquid within the reservoir.
It is a further object of the invention to provide a longitudinal groove within the reservoir to direct fluid sheared by the doctor blade in an arc to the surface of the moving transfer roll and thus reduce and control turbulent flow or cascading of wetting liquid within the reservoir.